A Digital Airport Surveillance System (DASR) system consists of two electronic subsystems; namely a Primary Surveillance Radar (PSR) and a Secondary Surveillance Radar (SSR). The PSR typically includes a continually rotating antenna mounted on a tower to transmit electromagnetic waves that reflect or backscatter from the surface of a target typically up to a radial distance of 60 nmi. The PSR also provides data on six levels of rainfall intensity. The SSR typically includes a second radar antenna attached to the top of the PSR to transmit and receive area aircraft data for barometric altitude, identification code, and emergency conditions. The air traffic control uses this system to verify the location of an aircraft within a typical radial distance of 120-nmi from the radar site.
In the tracking stage, data received from the PSR and SSR are combined based on the proximity of range and azimuth values of targets detected by the PSR and SSR. To meet the combination criteria which determines whether or not targets are combined, the range and azimuth differences of corresponding targets detected by the two sensors are calculated. If each parameter difference is within certain predefined limits, the targets are combined.
Additional tests on speed and heading are performed to resolve ambiguity in potential PSR/SSR target pairs. This is accomplished by examining speed and heading differences for the ambiguous targets to determine if the ambiguous targets should be combined. SSR data typically takes precedence over PSR data. The SSR range and azimuth values are used in a combined report unless the preferred radar is set to the PSR.
There are several issues with the DASR system. These issues include a requirement to remove erroneous tracks related to weather-related, anomalous propagation (AP) and ground clutters plots. Another requirement is to increase the air traffic security and safety level by identifying small or non-cooperative aircrafts without SSR data either due to the lack of a transponder, or by the accidental or deliberate disablement of the transponder. Another requirement is to identify bird, insects, and other biological tracks to avoid biological hazards to commercial and military aircrafts. Another requirement to increase air traffic safety by identifying tracks from helicopters, Unmanned Aerial Vehicles (UAV), etc.
Identifying biological and weather tracks and detecting aircrafts without SSR data are the most important and demanding challenges in the above items. Relying on the presence of SSR data for aircraft detection can impose serious risks to air traffic security and safety since the SSR system relies solely on transmission from onboard transponders to identify aircraft. Accordingly, only by correctly classifying non-transponder transmitting targets, can the presence of unknown aircraft be reported.
The increase in erroneous tracks due to weather clutter can also severely affect the radar performance in aircraft detection in adverse weather conditions. Tracks and echoes from objects such as buildings and hills may also appear on the radar display. Other examples of ground clutter include vehicles and windmills. This ground clutter generally appears within a radius of 20 nautical miles (nmi) of the radar as a roughly circular region with a random pattern. The AP phenomenon is another source of false tracks. These tracks occur under highly stable atmospheric conditions (typically on calm, clear nights), where the radar beam is refracted almost directly into the ground at some distance from the radar, resulting in an area of intense-looking echoes. Examples include certain sites situated at low elevations on coastlines that regularly detect sea return, a phenomenon similar to ground clutter except that the echoes come from ocean waves.
For biological targets such as birds, the impact is more serious. Echoes from migrating birds regularly appear during night-time hours between late February and late May, and again from August through early November. Return from insects is sometimes apparent during July and August. The apparent intensity and aerial coverage of these features is partly dependent on radio propagation conditions, but they usually appear within 30 nmi of the radar and for weather radar produce reflectivities of less than 30 dBZ.
The existence of birds in the vicinity of airport runways and flight paths present serious hazards to air traffic, particularly during the take-off, climb and landing approach when the loss of one or more engines can jeopardize flight safety. Birds are a worldwide problem in aviation. The danger and costs involved in biological strikes to aircrafts are enormous. Approximately 3000 wildlife strike incidents occur yearly to military aircraft and over 2200 wildlife strikes on civilian aircrafts in the US alone. Notably, the bird problem received greater emphasis in the US following the crash of an Airborne Warning and Control System (AWACS) aircraft in November 1995.
The images of bird echoes can completely fill all radar bins. Bird tracks, especially in coastal environments, can form a substantial proportion of the track population of a radar picture. The number of birds close to coastal roost sites can range from 10,000 to 1,000,000 birds, with similar densities possible within well-vegetated non-coastal areas. At peak migration periods, the number of airborne birds can reach 1,000,000 within a 50 km radius with many of these birds travelling in flocks.
Current airport surveillance radars, such as the ASR-9, intentionally reject bird tracks as unwanted clutter. Having known that birds typically fly much slower than aircrafts, changing the “velocity editor” is one method of eliminating bird tracks. However, it has been shown that this technique also removes primary targets with airspeeds below the set threshold, including helicopters and small non-transponder aircrafts. For this reason, a robust classifier with the ability to correctly identify different classes of targets is needed.